During the previous funding period of this grant we made two conceptually distinct but important observations. First, we found that the endocytic and post-endocytic trafficking of opioid receptors is critically important for the development of tolerance and dependence to opioid drugs. Second, we demonstrated that opioid receptors heterodimerize to form a unique target for a heterodimer-selective agonist. Here we will follow up on both of these observations by examining how receptor pharmacology and trafficking are altered by coexpression/heterodimerization of the opioid receptors with a focus on heterodimers containing the delta opioid receptor (DOR). Two distinct pharmacologically-defined subtypes of DOR have been described, although only a single gene has been identified. Although there are many explanations for this finding, one possibility is that co-expression or heterodimerization of the DOR with other opioid receptors alters the pharmacology to produce unique opioid receptor subtypes. We will examine how DOR subtype pharmacology is altered by deletion of individual opioid receptor types. In addition, we propose that the constellation of opioid receptor subtypes or heterodimers is altered following chronic morphine administration and the development of tolerance and dependence, possibly as a consequence of alterations in receptor trafficking. Changes in the distribution of opioid receptor subtypes or heterodimers could provide novel targets for the prevention or treatment of opioid tolerance and dependence. In addition, it is possible that certain opioid receptor subtypes or heterodimers are expressed in a tissue-selective manner, a gender-specific manner, or in a disease state-specific manner (for example in chronic pain states or following prolonged use of opiate drugs). Thus targeting these subtypes or heterodimers could provide improved therapeutic utility.